Conduct Disorder (CD) and Attention-Deficit Hyperactivity Disorder (ADHD) are among the most commonly diagnosed, referred, and treated disorders of childhood. Children with CD, ADHD, and CD+ADHD are at increased risk for antisocial behavior in adolescence and adulthood. While emotional dysfunction and physiological underarousal have been reported in CD and ADHD, psychophysiological techniques have only recently been utilized to investigate these abnormalities. Over the last decade, affective modulation of the startle reflex has been used to measure emotional and physiological reactivity in normal, psychiatric, and neurologically impaired individuals. Normal adults and infants show smaller startle reflex responses to pleasant stimuli and larger startle reflex responses to unpleasant stimuli. Adult criminal psychopaths fail to show this "normal" pattern of reactivity. Few studies have examined affective modulation of the startle reflex in normal children, and no study has examined it in children with CD or ADHD. The purpose of this 3-year project is to compare the emotional reactivity in four groups of children: CD, ADHD, CD+ADHD, and healthy controls. Physiological measures (startle magnitude, heart rate, skin conductance) and subjective judgments will be used to measure emotional reactivity to a standardized set of picture stimuli. The goals of this proposal are to (1) determine the patterns of emotional reactivity that discriminate among children with different behavioral disorders and (2) examine the relationship between emotional reactivity, impulsivity and symptoms of psychopathy. Results may lead to a better understanding of the physiological markers of emotional processing in two common childhood behavior disorders.